The present invention relates to pneumatic tires and the method of making such tires. More particularly, the present invention relates to the construction of a preform tire using an inner liner formed of a plurality of overlaying plies.
The automotive pneumatic tire is typically made using the flat band method of tire building wherein the various components, or plies, of the tire are applied, as flat stock, upon a rotating tire building drum forming a hollow, barrel shaped preform. In building the preform, the inner liner is first wrapped upon the drum followed by the carcass plies generally containing the tire reinforcement. The carcass plies are then followed by the belt plies, the side wall plies and the tread ply. The barrel shaped preform is then removed from the building drum and placed within a vulcanizing mold, having the general shape of the finished tire. The barrel shaped preform is next heated and expanded radially outward into the mold periphery typically by directing pressurized hot gas or steam into a bladder disposed within the preform. During the radial expansion of the preform within the mold, the cylindrically shaped plies must expand radially outward to radial dimensions much beyond those of the original preform. Therefore, the elastomeric material of each preform ply must stretchingly expand much like a balloon. During this expansion step the material thickness of the preform plies may be expected to decrease.
Of particular concern, in the present invention, is the inner liner ply. Typically the inner liner ply is laid upon the building drum as a flat piece of elastomeric stock having a uniform thickness. Thus, as the inner liner ply expands radially outward during the expansion step, the thickness of the inner liner is stretchingly thinned resulting in an inner liner ply having a thickness that progressively decreases from the tire bead area to the tire crown.
Therefore, to have a given inner liner thickness in the tire crown area, the thickness of the inner liner ply, when laid upon the tire building drum, must be oversized to allow for the stretching of the ply as the preform is expanded during the expansion step. Accordingly, the typical automotive tire structure exhibits an inner liner ply having excess material in the tire bead and side wall areas.
Prior art patent application WO-A-96 30221 also shows an air-permeation prevention layer (8) with layers (7) extending from the ends of a belt (6) toward the center (C).
To overcome the above described disadvantage, it has been a practice of the prior art to provide an elastomeric sheet having a variable thickness across its lateral width. Such inner liner sheet stock embodies a sheet thickness that progressively increases toward the center of the sheet from both sides. Thus, when such sheet stock is radially stretched during the expansion step, stretching occurs in the area of increased thickness thereby forming an inner liner having an approximate uniform thickness. However, such specialized sheet stock is more expensive than the uniform thickness sheet stock to manufacture and is unsuitable for rolling and storing on a feed spool prior to applying it upon the tire building drum.
It is an object of the present invention to provide a method and tire preform structure to eliminate the variable thickness of an automotive pneumatic tire inner liner when built by the flat band method of tire building.
Another object of the present invention is to provide a method and preform structure which results in a savings of elastomeric material in the tire inner liner that aids in weight reduction of a given tire.
In accord with the present invention, the standard practice of applying a single ply of elastomeric material upon the tire building drum to form the inner liner is replaced, in a first embodiment, by two overlapping plies of elastomeric material each having a uniform thickness. The two overlapping plies are laid upon the tire building drum such that when the tire preform is expanded radially outward during the expansion step, the gauge of the overlapping liners is reduced by stretching circumferentially to that necessary to prevent air penetration. Because of the minimal meridional (bead to bead) stretch during the expansion step, a single layer of the liner plies is sufficient.
As an alternative embodiment, the inner liner ply may be made-up of two separate elastomeric plies laid upon the building drum in a spaced relationship to each other with a third ply of elastomeric material being laid thereupon and overlapping the two spaced plies. As with the first embodiment, the three overlapping plies are laid upon the tire building drum such that when the tire preform is expanded radially outward during the expansion step, the gauge of the overlapping liners is reduced by stretching circumferentially to that necessary to prevent air penetration. Because of the minimal meridional (bead to bead) stretch during the expansion step, a single layer of the three liner plies is sufficient.